Non premixed combustion model ist best used for high temperatures (>1000°C) where the gas phase will usually reach equlibrium fast. For lower temperature prePDF will overpredict slow species like CO or NOx which are far away from equlibrium. In this case you should use some finite rate chmistry model for the gas phase. Btw even 15,000 ppm sounds very low for gasification...

you're saying 15000ppm is low for gasification? i'm not sure on this. in my case, the reactor is 1.87height with diameter of 1.8m. i run for 50kg of wood to avoid high temperature in the gasifier.i don't want the axial temperature to be higher than 1000 degree to avoid damage of type-K thermocouple (length of 1m).

The max. CO(ppm) is just around 2000 ppm near the gasifier outlet. there's only primary air in this gasifier. so, the product gases from this gasifier will then go the combustion chamber.

so, do i have to change all my method? can i use finite rate chemistry model with DPM?

Since gasification is the oxidation of Carbon to CO with the goal to produce alot of CO (and H2) 0.2 vol-% CO does not sound very high if your reactor is a real gasifier. If you just use the reactor for wood pyrolysis to produce charcoal and gas then the CO content can be low. In this case 2000 ppm sounds reasonable.

DPM can be used with finite rate chemistry. All you have to do is find a kinetic model that describes the CO formation during wood devolatilization/pyrolysis and the follwoing burnout. This wont be easy so you might consider developing your own kinetic mechanism based on your measurements.